Vascular aneurysms are produced when a thinning or weak spot in a vessel wall dilates, eventually posing a health risk from its potential to rupture, dissect, cause blood clots, or internal bleeding. While aneurysms can occur in any blood vessel, most occur in the aorta and peripheral arteries. The majority of aortic aneurysms occur in the abdominal aorta, usually beginning below the renal arteries and often extending into one or both of the iliac arteries. The etiology of aneurysm formation is not entirely understood, but is thought to be related to congenital thinning of the artery, atherosclerotic vessel degeneration, vessel trauma, infection, smoking, high blood pressure, and other causes leading to vessel degeneration. Left untreated, aneurysms may lead to gradual vessel expansion, thrombus formation leading to stroke or other vessel blockage, vessel rupture, shock, and eventual death.
Aneurysms may be treated in open surgical procedures, where the diseased vessel segment is bypassed and repaired with an artificial vascular graft. However, the surgical procedure is complex and requires experienced surgeons and well equipped surgical facilities. Patients suffering from such aneurysms are often elderly and weakened from cardiovascular and other diseases. This factor reduces the number of patients eligible for surgery. Furthermore, conventional aneurysm repair has a relatively high mortality rate, usually from 2 to 10%. Morbidity related to the surgery includes myocardial infarction, renal failure, impotence, paralysis, and other conditions. Even with successful surgery, recovery takes several weeks and often requires a lengthy hospital stay.
To overcome some of the drawbacks associated with open surgery, a variety of endovascular prosthesis placement techniques have been developed. Without the need for open surgery, patient complications and recovery time may be significantly reduced. One endovascular aneurism repair technique involves a tubular prosthesis deployed by remote insertion through a femoral artery. A stent-graft prosthesis permits a sealed shunt of blood flow from a healthy portion of the aorta, through the aneurysm, and into one or both of the iliac artery branches. The prosthesis excludes any thrombus present in the aneurysm while providing mechanical reinforcement of the weakened vessel reducing the risk of dissection and rupture. Furthermore, the prosthesis can substantially reduce the blood pressure within the isolated aneurysmal sac providing the weakened vessel with a favorable healing environment. Backflow from blood vessels in communication with the aneurismal sac may continue to pressurize the aneurysm despite the presence of a shunt.
A known shortcoming of some of the implantable endovascular prosthetics relates to migration and seal. The affected vessel(s) may vary widely in location, size, and the distended shape of the aneurysm itself. Particularly after treatment, the aneurysm and associated vessels may drastically change morphology thereby exerting stress forces on the deployed prosthesis. With sufficient change in aneurysm morphology and subsequent stress placed on the prosthesis, the device may migrate and/or detach from the vessel wall. As a result, the fluid seal may be compromised and blood may leak into the aneurysm from the aorta thereby elevating the aneurysmal pressure. The patient may have to undergo another treatment to prevent undetected “endoleakage” leading to aneurysm growth or regrowth, or other more serious problems associated with aneurysms. Accordingly, it would be desirable to provide a strategy for monitoring an aneurysm.
Current strategies for monitoring aneurysms involve imaging by means of CT-scan, magnetic resonance, angiography, duplex ultrasound, and the like. These imaging methods may utilize a contrast solution to enhance the visualization process. Some patients may be allergic to the iodine based contrast solutions and other “dyes”. In rare situations, the patient may suffer from anaphylactic responses involving mental confusion, dizziness (due to a drop in blood pressure), swelling (especially of the face, tongue and throat), and difficulty breathing. The reactions can be serious if not treated immediately. Therefore, it would be desirable to provide a strategy for monitoring an aneurysm without the use of appreciable volumes of contrast solution.
Another shortcoming of the aforementioned imaging strategies relates to sensitivity. Current methods may effectively visualize the size and shape of the aneurysm, providing a passive monitoring strategy. However, such methods may not effectively detect the presence of an endoleak. For example, if the aneurysm is largely filled with a thrombus, a sufficient amount of contrast solution may not be introduced into the aneurysm. This circumstance may lead to a reduced capacity to detect endoleakage. Continued undetected endoleakage, even at a low level, may slow or even reverse the aneurysmal healing process. To avoid this and other situations where endoleakage cannot be detected, it may be advantageous to measure endoleaks directly. Therefore, it is desirable to provide a sensing device and method for determining aneurismal pressure that overcomes the aforementioned and other disadvantages.
One such pressure-sensing device is disclosed in U.S. Pat. No. 4,846,191. Use of this device entails placing the tip of a pressure-transmitting catheter within a blood vessel or other structure within which pressure is to be measured. The catheter transmits the pressure signal to a transducer, which is typically connected to amplifying electronics and an implantable radio-transmitter capable of relaying the pressure information to a radio receiver located outside the body. Although such a device is capable of delivering pressure measurements from within the vasculature, it requires a complex device including a sizable implant in a body cavity.
Recently, miniaturized pressure sensors have been developed that can be placed directly into an aneurismal sac, and transmit pressure data to a receiving device outside the body. One such pressure sensor is disclosed in U.S. Pat. No. 6,159,156. This pressure sensor may be used in conjunction with an endoprosthesis and may be delivered with a catheter. U.S. Pat. No. 6,416,474 describes a sensor attached to a loop. The loop may encircle a tubular prosthesis, and may facilitate delivery of the sensor. The development of miniaturized sensors and other devices intended for use in the vascular system has created a need for a means of delivering them.
Various catheter-based systems have been developed to deliver medical devices and drugs to target sites within the body. U.S. Pat. No. 6,159,156 discloses use of a standard or specialized catheter for deploying a pressure sensor. The sensor is deployed by pushing it from the distal tip of the catheter, a process that may damage the sensor. U.S. Pat. No. 6,416,474 discloses a delivery system for a sensor attached to a loop. The delivery system includes a flexible catheter with a means of attaching the sensor via the loop to the distal end of the catheter, and an actuator for releasing the sensor that may be activated from the proximal end of the catheter. This delivery system is useful only for sensors or other medical devices that include a loop.
U.S. Pat. No. 6,447,522 discloses a delivery system for delivering tubular implants through the vascular system and placing them within tissue. One embodiment according to the invention includes a shaft with an outer tube that compresses and “crinkles” to a larger diameter in order to engage the inside surface of the implant. The outer tube can be extended to release the implant. Alternatively, the implants may be retained on the shaft by oval-shaped cross sections. A slidable cam within the shaft engages the oval areas, deforming them to a circular shape and releasing the implants. According to another embodiment, the implant is placed over a plunger that is driven by a pressurized fluid. This delivery system is clearly designed to deliver tubular or ring-shaped implants, and would not be useful for solid implants.
Recently, catheter-based delivery systems have been disclosed that include a sheath composed of a soft, pliable material, such as polyamide, polyurethane, polyimide, polytetrafluroethylene (PTFE), fluorinated ethylene propylene, or other medically acceptable polymers. To facilitate retraction, the sheath may be designed to be easily split along a longitudinal axis, and is referred to as a peelable sheath. Sheaths fulfill a variety of functions in catheter-based delivery systems such as allowing flexibility of the body of the delivery system, protecting the device being delivered, facilitating placement and retraction of the delivery device, and minimizing the potential for tissue injury. For example, U.S. Pat. No. 6,497,681 discloses a method for delivering a cardiac pacing lead or other elongated flexible device using a peelable introducer sheath and a device to cut or split the sheath longitudinally as it is retracted proximally and withdrawn from the body. In this instance, the peelable sheath facilitates retraction of the delivery device. U.S. Pat. No. 6,517,569 discloses a prostatic stent delivery device with a peel-away sheath that controls stent expansion during deployment, and can then be easily retracted. U.S. Pat. No. 6,533,806 discloses a delivery system including a balloon catheter and a sheath that surrounds and maintains an expandable prosthesis in a compressed condition until it arrives at the delivery site.
The above examples disclose highly specialized delivery systems that are not widely applicable for purposes other than those for which they were designed. Therefore, it would be desirable to provide a means of delivering a solid, spherical or oblong pressure sensor into the vascular system, especially into an aneurismal sac.